Intensity modulated light source

ABSTRACT

A device for producing intensity-modulated light over a broad frequency range comprising a frequency tunable light soruce, a control line for controlling the frequency of the light source to provide a first frequency at a first time and a second frequency at a second time, and an interferometer with a differential path length. The interferometer functions to recouple the light frequencies after propagating along different interferometer paths to effect a frequency beating. The resulting beat frequency is the intensity-modulated light.

BACKGROUND OF THE PRESENT INVENTION

The present invention relates generally to light modulators, and moreparticularly, to light modulators capable of generatingmicrowave-frequency light signals.

Present techniques for producing modulation of light generally make useof a controlled variation in the electrical signal energy supplied tothe light source, or make use of electrooptic or acoustoopticinteractions with light from a continuously emitting source. The maximummodulation frequency presently obtainable with these methods is a fewgigahertz at best, and such modulation rates in the gigahertz rangerequire significant driving circuits and microwave packaging. Thus, thecost size, electrical power dissipation, and complexity increase rapidlyfor modulation frequencies above approximately 100 MHz. By way ofexample, it is estimated that a source of modulated light with a 1 GHzbandwidth using state-of-the-art technology would cost more than$10,000, require an electrical drive power in excess of 10 watts, andcontain dozens of electronic and optoelectronic components. It has beensuggested to interfere or beat the light from two lasers to produce highfrequency intensity modulation at the difference frequency. However, itis difficult to stablize the laser frequencies to a degree adequate toinsure high spectral purity and low FM noise in the beat signal. In thisregard, see the article "Optical FSK Heterodyne Detection ExperimentsUsing Semi-Conductor Laser Transmitter and Local Oscillation", by S.Saito, Y. Yamamoto, and T. Kimura, IEEE J. Quantum Electronics, 1981,QE-17, pages 935-941.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the present invention to provide asimple and potentially inexpensive technique for producing intensitymodulation of light over a broad range of frequencies in the radiofrequency and microwave bands.

It is a further object of the present invention to provide an intensitymodulating device which contains only a few inexpensive solid statecomponents and has a tunability range in the tens of gigahertz.

Other objects, advantages, and novel features of the present inventionwill become apparent from the detailed description of the invention,which follows the summary.

SUMMARY OF THE INVENTION

The present invention describes a method and a means for generatingintensity modulated light by causing the frequency of a light source tovary with time and then interfering that light with light emitted by thesame light source at a different point in time.

Briefly, the present device for producing intensity modulated light overa broad frequency range comprises a frequency-tunable light source,circuitry for controlling the frequency of the light source with time ina desired manner, and an interferometer disposed to receive thefrequency varying light emitted from the light source and to interferethat light with light emitted from the light source at a different time,resulting in an intensity modulated light signal at the beat frequencyof the interfering light.

The interferometer may be realized in one embodiment by a Mach-Zehnderinterferometer using either partially reflecting mirrors as beamsplittercouplers and air as the propagating medium, or using one or moreevanescent-field fiber optic beamsplitter couplers with optical fibersfor the propagation paths. In a second embodiment the interferometer maybe realized with a four-port fiber coupler having a fiber coil connectedbetween one input and one output port.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of the intensity-modulated lightsource of the present invention.

FIG. 2 is a schematic diagram of one embodiment of theintensity-modulated light source using partially reflecting mirrors asbeamsplitters and air as the propagation medium.

FIG. 3 is a schematic diagram of a second embodiment of theintensity-modulated light source with evanescent-field fiber optic 3 dBcouplers as beamsplitters and optical fibers as the propagation paths.

FIG. 4(a) is a frequency versus time graph illustrating the generationof a signal-frequency output by means of a linear-chirped laser input.

FIG. 4(b) is a frequency versus time graph showing the generation of asingle-frequency pulse from a step increase in the laser inputfrequency.

FIG. 4(c) is a frequency versus time graph showing the generation of twosingle-frequency pulses from a step pulse in the input laser frequency.

FIG. 4(d) is a frequency versus time graph showing the generation of achirped output from a non-linear chirped laser input.

FIG. 5 is a third embodiment of the modulated light source of thepresent invention utilizing a single four-port coupler and fiber opticlight paths.

FIGS. 6(a), (b) (c) and (d) are drawings of photographs of a spectrumanalyser display of the output beat signal as a function of Ip-p from0.2 mA through 1.8 mA.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As noted above, the present invention describes a method and a means forgenerating intensity-modulated light by causing the frequency of the alight source to vary with time and then interfering, or beating, lightfrom that light source with light emitted from the same light source ata different point in time. Referring now to the drawings, wherein likereference characters designate like or corresponding parts throughoutthe views, FIG. 1 shows the present invention in its most basicconfiguration. Therein, a light source 10, which is capable of beingtuned in frequency over time, is coupled into an interferometer 12 withunequal optical path lengths. If the temporal variation in laserfrequency is described by the function v(t), then the optical outputbeams from the interferometer are intensity modulated at the beatfrequency f_(b) of the two interfering beams, given by the equationf_(b) (t)=v(t-T₁)-v(t-T₂), where T₁ and T₂ are the time delays for theoptical paths in the interferometer 12. It can be seen that the outputI₁ (t) on line 14 and the output I₂ (t) on line 16 will have thefollowing variation:

    I.sub.1 (t)=C.sub.1 +C.sub.2 cos (F.sub.b t+φ)

    I.sub.2 (t)=C.sub.1 +C.sub.2 cos (F.sub.b t+φ)

where C₁, C₂, and φ are constants.

The basic device shown in FIG. 1 may be implemented in a variety ofconfigurations. FIG. 2 shows one such configuration wherein the lightsource 10 is implemented by a laser and the interferometer 12 isimplemented by a Mach-Zehnder interferometer. A Mach-Zehnderinterferometer typically comprises two optical beamsplitter couplers.The first beamsplitter divides the input optical power into two beamswhich traverse optical paths of different lengths. The secondbeamsplitter then recombines the beams to produce a beat signal. In FIG.2 the Mach-Zehnder interferometer comprises series of reflecting andpartially reflecting mirrors with air as the propagation medium. Thelight from the laser source 10 is directed at a first partiallyreflecting mirror 20 which acts to split the light beam to propagatealong two paths 22 and 24. One of these light paths is significantlylonger than the other path, although this has not been represented inFIG. 2 for purposes of convenience. The light path 22 includes a mirror26 for reflecting the light propagating thereon to one side of apartially reflecting mirror 28. Likewise, the path 24 contains a mirror30 for reflecting the light propagating on this path to the other sideof the partially reflecting mirror 28. The partially reflecting mirror28 acts to recombine and mix the beams to yield the modulated signals I₁(t) on line 14 and I₂ (t) on line 16.

FIG. 3 shows a second configuration for the basic invention, Therein theinterferometer is again realized by a Mach-Zehnder interferometerutilizing evanescant-field fiber optic beam splitters with opticalfibers for the propagation paths. The light from a laser source 10 isdirected to the input face 42 of an optical fiber 44 by means of a lens40. The light propagating on this optical fiber 44 is then split intotwo beams by means of a 3 db coupler 46. This 3 db coupler 46 may berealized in the well-known manner simply by disposing a second opticalfiber 46 in parallel proximity to the first optical fiber 44 in order toobtain evanescent-field coupling. Again, one of these two optical fiberpaths 44 and 46 is significantly longer than the other. In FIG. 3, theoptical fiber path 46 includes a coil 48 of fiber wound on a reel tomake it the longer path. The optical fiber paths 44 and 46 are thenbought into parallel proximity to form a second 3 db evanescent-fieldcoupler 50 to mix the light propagating in these paths and therebygenerate the light outputs I₁ (t) and I₂ (t) intensity modulated at thebeat frequency f_(b).

The optical laser source of FIG. 2 and FIG. 3 may be realized by anytype of tunable laser such as, for example, a tunable dye laser, or atunable semiconductor diode laser. Best performance will require the useof lasers which emit in a single mode to give a high degree of spectralpurity. Spectral purity is desired in order to obtain a narrow laserline to control the width of the modulated beat frequency signal. Thepresently described process entails the beating of two spectra from thesame laser together such that the convolution of the two waveforms willdetermine the spectral width of the beat frequency signal generated.

The semiconductor diode laser appears particularly attractive from apractical standpoint because of its small size, low cost, highelectrical-to-optical conversion frequency, and rapid tunability over awide range of frequencies. Frequency tuning of the diode laser can beaccomplished by varying the ambient or heat-sink temperature or byvarying the diode current. It is noted that a typical semiconductordiode laser in thermal equilibrium with its heat sink changes infrequency at a rate of about 1 GHz per mA change in current. Thus, thisstatistic gives some idea of the magnitude of the tuning which can beobtained by varying the current to the diode. As an example, it can beseen that only a 10 mA change in driving current is required to obtain a10 GHz frequency change in the intensity modulation of theinterferometer output.

A wide variety of single-frequency, chirped, pulsed, and frequencyhopping waveforms can be obtained, depending on the temporal variationin the laser frequency. Some of these waveforms are illustrated in FIG.4. More specifically, in FIG. 4(a), there is shown the generation of asingle frequency output f_(b) (t) by means of a linear-chirped laserinput. In FIG. 4(b) the generation of a single-frequency pulse f_(b) (t)is shown by means of a step increase in the input laser frequency. InFIG. 4(c) the generation of two single-frequency pulses is illustratedby means of a step pulse in the input laser frequency. In FIG. 4(d) thegeneration of a chirped output signal is illustrated by means of anon-linear chirped laser input.

An experimental set-up for the device is shown in FIG. 5 and comprises athird embodiment of the present invention. This configuration comprisesa laser diode 10 as the light source and a four-port evanescent-fieldfiber coupler 60 in combination with a length of single-mode fiber 62 asthe interferometer. The length of optical fiber utilized in theexperiment was 1.1 km which provided a 5.5 us time delay. The device isconnected as follows. Light from the laser diode 10 is applied via anoptical fiber 64 into one of the input ports of the coupler 60. Thelength of optical 62 is connected between one of the output ports of thecoupler 60 and the second of its input ports. Thus, light from the laserinjected into the first input port is coupled into both output portssuch that part of the laser light propagates through the optical fiber62. After propagation through the fiber 62, the light enters the secondinput port of the coupler 60, where it mixes with the undelayed emissionfrom the laser applied on the optical fiber 64. Light from the otheroutput port of the coupler 60 is focused in the experiment onto aphotodetector 66, which may be realized by a silicon avalanchephotodiode with a response bandwidth of 2 Ghz. The photodiode 66produces an electrical signal at the difference frequency for thedelayed and undelayed laser emission. The microwave spectrum of thisdifference frequency was then observed on a Hewlett Packard spectralanalyzer 68.

It should be noted that a beat frequency component will also be coupledback into the fiber coil 62. However, this beat frequency component willbe significantly weaker than the most recently coupled signalpropagating therein due to attenuation of the beat signal in the opticalfiber and in the four-port coupler 10. This beat frequency component isnot present in the Mach-Zehnder interferometer configurations of FIG. 2and FIG. 3.

In the experiment actually performed, a single mode CSP GaAlAs laserdiode with a dc bias current 20-30 percent above threshold was used asthe laser source. Frequency tuning of the laser was accomplished bysuperimposing a time-varying current I(t) 70 through a bias tee.Time-dependent changes in the emission frequency v(t) of a laseroperating under various modulating conditions are shown in the article"Microwave Signal Generation Using An Optical Self-Heterodyne Technique"by L. Goldberg, J. F. Weller, H. F. Taylor, Electronics Letters, Apr.15, 1982, Vol. 18, No. 8, pages 317-319. Frequency tuning in the laserdiode is due to both changes in carrier density and in the laser diodetemperature, with the thermal effect dominant for times greater than 100ns after the beginning of a current pulse.

As noted above, to obtain a spectrally narrow microwave signal with lowFM noise, the laser emission line width should be small. Also, thecenter frequency should be constant over most of the current modulationcycle. To narrow the laser emission line width, optical feedback from anoptical cavity is used. In the embodiment shown in FIG. 5, a mirror 72is located 60 centimeters from the laser facet in order to provide afeedback signal on the order of 10⁻⁴. A lens 74 is utilized to focus thelight from the laser diode on to the feedback mirror 72. This feedbackcauses the laser line to narrow considerably and the center frequency tovary in stepwise fashion as a function of current. The time dependenceof the frequency for a square-wave modulated laser operating withfeedback is shown in FIG. 2(b) of the aforementioned Goldberg article.Discrete jumps of 250 MHz corresponding to the frequency spacing of theexternal modes given by C/2L, were observed. Between these jumps, theemission frequency remained relatively constant. To achieve more abrupttransitions in v(t), prebias pulses may be superimposed on the squarewave so that the laser temperature and emission frequency are brought totheir equilibrium levels more quickly. With the addition of feedback,the v(t) of the laser more closely approaches the ideal square-wavepattern.

The actual output of the microwave spectrum analyzer 68 is shown in FIG.6. A square-wave at 0.5 GHz is used as the modulating waveform with aperiod of 11.0 μs. The prepulse amplitude and length 10 were I_(p-p) and0.3 μs, respectively. The spectrum analyzer display of FIG. 6 indicatesthat the diode photodetector output occurs in a single dominant peak v₀,and with smaller signals at v₀ ±250 MHz. These additional peaks are dueto the fact that in the presence of feedback the laser emission spectrumcontains several weak external cavity modes in addition to the singledominant mode. By changing the I_(p-p), the v₀ is tuned at a rate of 1.0GHz/mA. Since with feedback the laser emission frequency varies in astepwise fashion as a function of current, only integral multiples of250 MHz are observed in the photodetector output signal.

In conclusion, a simple technique for obtaining optical beat signals atmicrowave frequencies using a tuned diode laser has been disclosed. Anarrow microwave spectrum may be obtained by feeding a portion of thelaser emission back into the cavity. With proper current tuning, itshould not be difficult to obtain modulation at several tens ofgigahertz by this technique.

The primary advantages of the present intensity modulation techniqueinclude low cost, small size, low electrical power dissipation, andbroad frequency tuning range. Applications of this technique areforeseen in transmitters for optical fiber communications and in thegeneration of microwave signals for radar and electronic warfare. Incommunications, the intensity modulated light source would be an idealFM transmitter for either analog or digital transmission, since themodulation could be accomplished with milliwatt changes in electricalcurrent. An equivalent system with conventional components would requirecomplex and expensive oscillators and switches, and dissipate orders ofmagnitude more electrical power.

It is also noted that when a photodetector is used with this device, theoptical signal generated thereby can be converted to a microwave signal,which can then be amplified and transmitted. The frequency of theintensity modulated source can be varied over tens of gigahertz in timesmuch less than one microsecond, thereby providing a versatile techniquefor producing complex chirped, pulsed, and frequency hopping waveformsfor radar transmitters and for jamming enemy signals in electronicwarfare.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. A device for producing intensity modulated lightover a broad frequency range comprising:a frequency-tunable lightsource; a control line for varying the frequency of said light sourcewith time in a desired manner such that light of a first frequency f₁ isprovided by said light source at a first time and light of a secondfrequency f₂, different from said first frequency, is provided by saidlight source at a second time; and an interferometer with differentoptical path lengths disposed to receive said first frequency light fromsaid light source and to interfere that light with said second frequencylight emitted from said light source at said second time, resulting inan intensity modulated light signal at the beat frequency of |f₁ -f₂ |.2. A device as defined in claim 1, wherein said light source is atunable laser.
 3. A device as defined in claim 2, wherein saidinterferometer is an Mach-Zehnder interferometer comprising:a firstoptical path; a second optical path with a longer optical length lengththen said first optical path; a first beamsplitter for splitting thelight beam from said light source to propagate along said first andsecond optical paths; and a second beamsplitter for combining the beamsafter they have traversed said first and second optical paths to yield aresultant intensity modulated light signal.
 4. A device as defined inclaim 2, wherein said interferometer is a Mach-Zehnder interferometercomprising:a first optical fiber path; a second optical longer fiberpath with a longer length then said first optical fiber path; a first 3dB coupler for coupling light from said light source into said first andsecond optical fiber paths; and a second 3 dB coupler for coupling thelight after propagating through said first and second optical fiberpaths to yield a resultant intensity modulated light signal.
 5. A deviceas defined in claim 2, wherein said interferometer comprises:a four-portevanescant field fiber coupler with an input first port connected tosaid light source, with an output third port connected to an outputline; and an optical fiber coil connected between an input second portand an output fourth-port of said four-port fiber coupler.
 6. A deviceas defined in claim 5, further comprising a transducer connected to theoutput line for converting light propagating on said output line to anelectrical signal at the beat frequency of the interfering beams.
 7. Amethod of generating intensity-modulated light over a broad frequencyrange comprising the steps of:temporally varying the frequency of alight source in a predetermined manner to yield a first frequency f₁ ata first time and to yield a second frequency f₂, different from f₁, at asecond time; and interfering the light at frequency f₁ emitted from thelight source at said first time with the light at frequency f₂ emittedfrom the light source at said second time to yield an intensitymodulated light signal at the beat frequency of the interfering light.8. A method as defined in claim 7, wherein said interfering stepcomprises the steps of;splitting the light emitted from the light sourceinto two beams which propagate on different paths; delaying the lightpropagating on one path relative to the other path; and coupling thelight on the two differentially delayed paths together to yield aresultant light output.
 9. A device as defined in claim 3, wherein saidfrequency-tunable light source laser includes means for applying aprebias pulse to said laser so that the laser temperature and emissionfrequency are brought to their equilibrium levels quickly.
 10. A deviceas defined in claim 4, wherein said frequency-tunable light source laserincludes means for applying a prebias pulse to said laser so that thelaser temperature and emission frequency are brought to theirequilibrium levels quickly.
 11. A device for producingintensity-modulated light over a broad frequency range, comprising:asingle frequency-tunable laser; a control line for varying the frequencyof said laser with time to obtain light of a first frequency f₁ at afirst time and light of a second frequency f₂, different from said firstfrequency, at a second time; delay means for delaying light of saidfirst frequency emitted from said single laser at said first time; andmixing means for mixing the delayed light of said first frequency fromsaid delay means with said light of said second frequency obtaineddirectly from said single laser, resulting in an intensity-modulatedbeat signal.
 12. A device as defined in claim 11, wherein saidfrequency-tunable light source laser includes means for applying aprebias pulse to said laser so that the laser temperature and emissionfrequency are brought to their equilibrium levels quickly.